Many television programs are originally shot with film. For National Television Standards Committee (NTSC) video distribution, the film source material (at 24 frames/s) is converted to video medium (at 60 fields/s) before editing. The most common method of converting film source material to NTSC video medium involves converting one film frame into either two or three video fields using a repeating 3:2 field pattern, referred to as a 3:2 pulldown, as shown in FIG. 1. One film frame on an input device, typically a film gate, is transferred to two video fields on an output device. The next film frame on the input device is then transferred to the next successive three video fields, the first and the third field being duplicates. The third input film frame is transferred to the successive two video fields on the output device and so on. Thus, a single film frame may correspond to either two or three successive video fields. The redundant video field may cause slight "motion artifacts" when the video is played back. Fortunately, the effect of this is minimal and the additional fields do not appreciably affect the smoothness of the material for most viewers. Once the source material has been transferred to video, the video material is edited, then distributed. Editing the video material affects the 3:2 pattern, making it difficult to return the video to its original source material frame rate.
The additional video fields do, however, present a problem in other instances. Recent developments in digital compression technology have enabled conversion, storage, and transmission of this video material in the digital domain. In order to allow video to be delivered over existing computer and telecommunications networks, video sources have to be converted to an optimal format for compression. When compressing video that was originally film source, the presence of duplicate fields results in sub optimal compression. Duplicate video fields are compressed, thus reducing the efficiency of the compression. The highest efficiency compression can be achieved when the redundant information is skipped and the frame rate is returned to that of the original film source.
The ideal solution may seem to be a film-to-compressed video conversion apparatus that can avoid all the problems associated with returning the material to its original frame rate. This solution presents a number of problems however. First, no such apparatus has yet been created. Additionally, editing is best done before compression, so this solution would require film editing that is much more expensive than video editing. Finally, the film and video industries have already invested large amounts of money into film-to-video conversion equipment and video editing equipment. Thus, an optimal solution for returning the video material to the source material frame rate before compression will require a reverse of the 3:2 pulldown process described above. This conversion, hereafter referred to as inverse 3:2 pulldown, is shown in FIG. 2.
No practical conversion methods currently exist in the industry to perform the inverse 3:2 pulldown and return the video frame rate to its original source rate for optimal compression. Methods exist to detect video signals containing images that have been converted from film source. For example, U.S. Pat. No. 5,317,398 uses circuitry for generating pixel value differences in successive frames of the video signal. The differences are then accumulated and input to a correlation circuit. Average values, known as correlation values, are also computed and subtracted from the output of the correlation circuit. The video signal is determined to have been created from film source if the difference values are greater than a predetermined threshold value. This method does not, however, generate an output of what the 3:2 pulldown cycle is during a respective frame interval of the video signal. This 3:2 pulldown cycle is crucial in inverse frame rate conversion to the original film rate without creating annoying stutter in any motion of the video. These methods thus do not provide the required information for restoring the original film frame rate.